The use of directed plasmas as a thruster for accelerating a mass on which the plasma source is located has been previously suggested. In Mastrup, U.S. Pat. No. 3,447,322, is disclosed a pulsed ablating thruster apparatus wherein an electric discharge is established in a passageway having one end terminated in a nozzle. The passageway is a bore of an ablating thruster body formed of material such as Plexiglass, nylon and Teflon. In response to a discharge voltage being established between opposite ends of the passage, material is ablated from a confining wall of the passage. The ablated material flows out of the nozzle to provide an accelerating force for the mass on which the thruster is located. Mastrup indicates that the pressure in the passage is to be maintained below 200 atmospheres.
There are numerous problems associated with the thruster disclosed in the Mastrup patent. In particular, ablating the types of materials specifically mentioned in the patent results in a relatively low thrust efficiency of about 20%. This is because of the low pressure of less than 200 atmospheres, which causes the gaseous materials which are formed by the ablation process in response to the discharge voltage to be emitted in a highly ionized and dissociated state. In addition, the confining surface, i.e., wall, of the ablated passage becomes excessively hot if attempts are made to activate the thruster repetitively to generate high thrust, i.e., the product of mass flow rate and velocity. Also, the maximum pressure of 200 atmospheres is insufficient to obtain high thrust in a small, compact device.
A further electric thruster, disclosed in LaRocca, U.S. Pat. No. 3,575,003, includes a wax-like working substance for electrically powering thrust engines by prolonged heating in a vacuum of liquid or soft grease made of fluorocarbon polymers. The resulting material flows under surface tension through tapered passages, moving slowly in a wax-like condition. When heated, the substance moves more rapidly and becomes more fluid. The substance ablates in response to the electric energy and flows out of a central cylindrical or conical aperture to provide propulsive forces to a mass on which the thruster is located. The thrust and efficiency which can be achieved with the structure of the LaRocca device are insufficient for many purposes to achieve any substantial payload velocity.
In the copending, commonly assigned applications of Goldstein et al, Ser. No. 471,215, filed Mar. 1, 1983, and Goldstein et al, Ser. No. 657,888, filed Oct. 5, 1984, are disclosed plasma propulsive systems wherein a projectile is accelerated in response to a plasma discharge established in a capillary passage. A discharge voltage is established between opposite ends of the passage to cause a plasma to be directed out of one end of the passage. The plasma acts on a projectile, to accelerate the projectile in the direction of plasma flow. In the invention of Ser. No. 471,215, the plasma is directed into a barrel through which the projectile is accelerated by a number of capillaries that are offset with respect to the barrel. In the invention of Ser. No. 657,888, the capillary passage and barrel are aligned. To reduce heating of the barrel, a fluent material, such as water, is located immediately downstream of the capillary passage. The fluent material mixes with the plasma ejected from the capillary passage to reduce the plasma temperature and prevent substantial ablation of the barrel.
The apparatus and method disclosed in the inventions of Ser. Nos. 471,215 and 657,888 are particularly advantageous because of the high plasma momentmm obtained each time a plasma jet is derived from the capillary passage. The high momentum occurs because of the very efficient transfer of energy from an electric power supply to the low molecular weight material used to form the plasma.
In one preferred embodiment in the inventions of Ser. Nos. 471,215 and 657,888, the plasma is formed by ablating hydrogen and carbon atoms from a wall of a polyethylene sleeve having an interior bore that forms the capillary passage. The low atomic weight of the carbon and hydrogen, as well as the electrical characteristics of the capillary passage and the flow characteristics of the passage, provides a highly efficient transfer of electrical energy into plasma kinetic energy.
We have found through experimentation that with appropriate modification structures of the type disclosed in the inventions of Ser. Nos. 471,215 and 657,888 can provide highly efficient thrusters capable of producing substantial thrust impulse. Modification of the structures disclosed in the Goldstein et al inventions for the thruster application is necessary because the capillary passages would become excessively hot if they were activated with the relatively high frequency required to provide the necessary thrust. In addition, there are other aspects of thruster operation, associated with efficient transfer of electric energy to plasma kinetic energy, which have no analogous counterparts in the use of plasmas to propel projectiles.
It is, accordingly, an object of the present invention to provide a new and improved plasma thruster.
Another object of the invention is to provide a new and improved highly efficient electrothermal thruster employing a capillary passage in which a plasma discharge is established.
Still another object of the invention is to provide an electrothermal thruster employing a capillary in which a plasma discharge is established and wherein the plasma discharge provides a relatively high thrust by virtue of relatively high repetition rate plasma discharges.
Still another object of the invention is to provide a new and improved plasma thruster employing a capillary passage in which plasmas are repeatedly developed at a high repetition rate and wherein a confining surface of the capillary passage is cooled.
Still another object of the invention is to provide a thruster employing a capillary passage in which a plasma discharge is repeatedly established at a high repetition rate and wherein a confining surface of the passage is cooled by a substance which adds to the momentum provided by the discharge.
A further object of the invention is to provide an electrothermal thruster employing a capillary in which a plasma discharge is established and wherein the plasma discharge provides a relatively high momentum by virtue of relatively high repetition rate plasma discharges that are converted into a quasi-continuous stream of directed mass from one end of the capillary.